AT THE CONCLUSION of the Strategic Economic Dialogue on July 28, the United States and China issued a news release affirming "the importance of the rule of law to our two countries." One day later, Chinese police led prominent legal scholar Xu Zhiyong out of his apartment to be detained indefinitely.

Few people embrace the rule of law in China as openly or as wholeheartedly as Mr. Xu. After graduating from one of China's most prestigious law schools, he has dedicated his life to fighting for justice by means of the Chinese legal system. He has represented the parents of more than 300,000 children affected by melamine-contaminated milk, opposed secret "black jails" and fought for the rights of death row inmates. Mr. Xu is a strong proponent of working within the system for change -- so much so that he ran for office in one of China's rare contested elections and won. Along with his colleagues at the Open Constitution Initiative, which he helped establish, Mr. Xu is a highly visible legal figure to whom people have increasingly turned as they gained awareness of their constitutional rights.

Nongovernmental organizations such as the Open Constitution Initiative, or Gongmeng, occupy an uneasy place in China. Many register as businesses to avoid dealing with a system that limits the number of NGOs and that requires government agencies to oversee their operations. This decision leaves them in a legal gray area, and in July, Chinese authorities charged that Gongmeng was improperly registered and had failed to pay taxes. Officials entered the organization's offices and confiscated dozens of files.

Mr. Xu was confident that he could defend his organization against these allegations by his usual means: through the system. But scarcely a week before he was to appear in court, police took him from his home.

He is not the first to see his trust in the system backfire. Last month, China refused to renew the licenses of 53 human rights lawyers whose cases troubled the Communist Party. But by taking on a figure as public and as scrupulously law-respecting as Mr. Xu, the Chinese government has crossed a new line. China frequently affirms its commitment to the "rule of law." But because the Chinese judiciary is not independent -- its chief justice is a longtime party member who lacks a law degree -- and court decisions often depend upon policy calculations, it is thanks only to the tireless efforts of those such as Mr. Xu that the "rule of law" might come to mean anything at all.

According to Chinese practice, Mr. Xu can be held for 30 days while the government decides whether to press charges; during this interval, protests from abroad might have some impact. In February, Secretary of State Hillary Rodham Clinton said that human rights issues should not "interfere" in the discussion with China. But engaging with China does not mean abdicating the responsibility to strenuously object to its human rights violations -- especially in such an egregious case as the detention of Mr. Xu.